1. Technical Field
The present disclosure relates to system(s) and method(s)/process(es) for fluidizing nanoparticles and nanoagglomerates. More particularly, the present disclosure is directed to systems and methods/processes for fluidizing nanoparticles and nanoagglomerates utilizing a fluidizing gas with one or more external forces, e.g., a vibration force, a magnetic force, an acoustic force, a rotational force and combinations thereof. Advantageous results are achieved, at least in part, by establishing a desired nanoparticle/nanoagglomerate particle size distribution within the system and substantially maintaining such distribution as the system achieves and maintains a fluidized state.
2. Background of Related Art
Fluidization is a widely used process in several industries to achieve continuous powder handling ability, particle mixing, and desirable levels of solid-gas contact. By definition, gas fluidization is a process in which solid particles are transformed into a fluid-like state through suspension in a gas. Gas fluidization is one of the best techniques available to disperse and process powders belonging to the Geldart group A and B classifications. Fluidization processes can be used to achieve high heat and mass transfer and reaction rates. Gas fluidization of small solid particles has been widely used in a variety of industrial applications because of its unusual capability of continuous powder handling, good mixing, large gas-solid contact area and high rates of heat and mass transfer.
Extensive research has been done in the area of gas fluidization, and the fluidization behavior of classical powders in the size range of 30 to 1000 μm (Geldart group A and B powders) is relatively well understood. However, the fluidization behavior of ultrafine particles, including nanoparticles which are in the extreme low end of Group C particles (<20 microns) in Geldart's Classification of Powders, is much more complex and has received relatively little attention in the literature. Nanoparticles are difficult to fluidize due to their strong interparticle forces. A bed of nanosized silica, for example, will exhibit plug formation, channeling, and/or spouting in a conventional fluidized bed. As far as is known, fluidization of nanoparticles (which are three orders of magnitude smaller than traditional group C powders) has heretofore been extremely difficult, if not impossible, to effectively achieve.
At least in part based on their very small primary particle size and very large surface area per unit mass, nanostructured materials are effective for the manufacture of drugs, cosmetics, foods, plastics, catalysts, high-strength or corrosion resistant materials, energetic and bio materials, and in mechatronics and micro-electro-mechanical systems (MEMS). Based on such uses, processing technologies which can handle large quantities of nanosized particles, e.g., mixing, transporting, modifying the surface properties (coating) and downstream processing of nanoparticles to form nano-composites, are desirable. But before processing of nanostructured materials can take place, the nanosized particles have to be well dispersed.
Strong interparticle forces exist between nanoparticles, such as van der Waals, electrostatic and moisture-induced surface tension forces. Based on such forces, nanoparticles are found to be in the form of large-sized agglomerates (rather than as individual nano-sized particles) when packed together in a gaseous medium. Hence, gas fluidization of nanoparticles generally refers to the fluidization of nanoparticle agglomerates.
It is generally possible to fluidize nanoparticles as relatively large agglomerates when the gas velocity exceeds the expected minimum fluidization velocity of the agglomerates. However, there tends to be significant powder loss and non-uniform fluidization behavior. In addition, large agglomerates can form near the distributor. Thus, there remains a need for a fluidization process that minimizes or avoids powder loss and accomplishes a smoother, more controlled fluidization with good mixing.
Previous studies of gas fluidization of nanoparticle agglomerates have found that the minimum fluidization velocity is relatively high (about several orders of magnitude higher than the minimum fluidization velocity of primary nanoparticles). The size of the fluidized nanoparticle agglomerates is typically from about 100 to 700 μm, while the primary particle size ranges from 7 to 500 nm. A typical nanoparticle agglomerate size distribution (by weight percentage) for a commercially available product (Aerosil® R972 silica; Degussa; Dusseldorf, Germany) is shown in FIG. 1. The data reflected in FIG. 1 was generated by: (i) randomly sampling a storage bag of commercially available R972 silica (20.0 g), (ii) sieving the sample using ten (10) different sieve sizes and measuring the weight retained on each such sieve, (iii) recording sieve size and particle weight, and (iv) calculating weight percentage for each sieve and plotting results. As shown on FIG. 1, a typical agglomerate size distribution for a commercially available nanoparticles products is widely dispersed and includes a significant weight percentage at larger agglomerate sizes.
For some nanoparticles, very smooth fluidization occurs with extremely high bed expansion, practically no bubbles are observed, and the velocity as a function of voidage around the fluidized agglomerates obeys a modified Richardson-Zaki equation. This type of fluidization of nanoparticle agglomerates has been termed agglomerate particulate fluidization (APF) by Wang et al [See, Wang et al., Fluidization and agglomerate structure of SiO2 nanoparticles, Powder Technology, 124 (2002) 152-159.8]. For other nanoparticles, fluidization results in a very limited bed expansion, and large bubbles rise up very quickly through the bed. This type of fluidization has been termed agglomerate bubbling fluidization (ABF) by Wang et al. However, even for the homogeneously fluidized nanoparticles, relatively large powder elutriation occurs at the high gas velocities required to fluidize the nanoagglomerates. This loss of particles may hinder the applicability of fluidization of nanoparticle agglomerates in industrial processes.
In addition to conventional gravity-driven fluidization, nanoparticle agglomerates can also be fluidized in a rotating or centrifugal fluidized bed [See, Matsuda et al., Particle and bubble behavior in ultrafine particle fluidization with high G, Fluidization X, Eng. Found, 2001, 501-508; Matsuda et al., Modeling for size reduction of agglomerates in nanoparticle fluidization, AIChE 2002 Annual Meeting, Nov. 3-8, 2002, Indianapolis, Ind., 138e], where the centrifugal force acting on the agglomerates can be set much higher than gravity.
A number of studies dealing with modeling and numerical simulation of the fluidization of nanoparticle agglomerates can be found in the literature. These models are based either on force or energy balances around individual agglomerates, the use of the Richardson-Zaki equation, or a combination of the Richardson-Zaki equation with fractal analysis for APF fluidization, or a modified kinetic theory. Recently, some applications of nanoparticle agglomerate fluidization were investigated, including the production of carbon nanotubes, and its application to photocatalytic NOx treatment. However, very little experimental data on the fluidization characteristics and differences between APF and ABF nanoparticles, such as minimum fluidization velocity, agglomerate size, hysteresis effects, and the effect of nanoparticle material properties, are available.
Sound waves, in combination with vibration, have been used to increase fluidization quality in cohesive powders whose sizes range from submicron to 20 microns. Also, vibration combined with gas flow has been used to successfully fluidize particles of smaller size, such as nanoparticles. However, notwithstanding the benefits associated with these known fluidizing techniques, often a dense immobile phase forms at a bottom of a fluidizing bed.
U.S. Pat. No. 4,720,025 to Tatevosian discloses a technique that utilizes an alternating magnetic field along with magnetic particles to loosen up material at the bottom of a hopper for feeding into a certain operation. However, the disclosed technique does not include loosening up cohesive materials for application in a fluidized bed. Similarly, U.S. Pat. No. 6,471,096 to Dave discloses the use of alternating magnetic field along with permanent magnets to produce controllable discharge of cohesive powders from a container, but does not provide for fluidization of nano-powders. U.S. Pat. No. 3,848,363 to Lovness et al. discloses the use of magnetic force to move particles in a predetermined area, but again does not provide for any application to fluidization.
The idea of using a magneto fluidized bed was proposed in 1960 by Fillipov [see, M. V. Filippov, The effect of a magnetic field on a ferromagnetic particle suspension bed, Prik. Magnit. Lat. SSR, 12 (1960) 215] and became popular as a means of suppressing bubbles in gas fluidized beds for a variety of industrial applications [see, R. E. Rosensweig, Process concepts using field stabilized two-phase flow, J. of Electrostatics, 34 (1995)163-187]. Generally, the particles to be fluidized were either magnetic particles or a mixture of magnetic and non-magnetic particles, and the magnetic field was usually generated by DC current [see; V. L. Ganzha, S. C. Saxena, Heat-transfer characteristics of magneto fluidized beds of pure and admixtures of magnetic and nonmagnetic particles, Int. Journal of Heat Mass Transfer, 41(1998) 209-218; J. Arnaldos, J. Casal, A. Lucas, L. Puigjamer, Magnetically stabilized fluidization: modeling and application to mixtures, Powder Technology, 44(1985) 57-6224; W. Y. Wu, A. Navada, S. C. Saxena, Hydrodynamic characteristics of a magnetically stabilized air fluidized bed of an admixture of magnetic and non-magnetic particles, Powder Technology, 90(1997) 39-46; W. Y. Wu, K. L. Smith, S. C. Saxena, Rheology of a magnetically stabilized bed consisting of mixtures of magnetic and non-magnetic particles, Powder Technology, 91(1997) 181-187; X. Lu, H. Li, Fluidization of CaCo3 and Fe2O3 particle mixtures in a transverse rotating magnetic field, Powder Technology, 107(2000) 66-78], causing magnetic particles to form chains along the field. For example, Arnaldos et al [see, J. Arnaldos, J. Casal, A. Lucas, L. Puigjamer, Magnetically stabilized fluidization: modeling and application to mixtures, Powder Technology, 44(1985) 57-6224] studied the fluidization behavior of a mixture of magnetic and non-magnetic particles of several hundred microns in size, such as sintered nickel-silica, steel-copper and steel-silica particles. The fluidization of larger particle mixtures of millimeter size (Geldart group D particles), such as iron-copper shot of 0.935 to 1.416 mm in diameter is described in [W. Y. Wu, A. Navada, S. C. Saxena, Hydrodynamic characteristics of a magnetically stabilized air fluidized bed of an admixture of magnetic and non-magnetic particles, Powder Technology, 90(1997) 39-46] and [W. Y. Wu, K. L. Smith, S. C. Saxena, Rheology of a magnetically stabilized bed consisting of mixtures of magnetic and non-magnetic particles, Powder Technology, 91(1997) 181-187], and Lu et al [X. Lu, H. Li, Fluidization of CaCo3 and Fe2O3 particle mixtures in a transverse rotating magnetic field, Powder Technology, 107(2000) 66-78] studied the fluidization of very fine (Geldart group C) particle mixtures of CaCO3—Fe2O3 in a transverse rotating magnetic field. However, in all of these studies, the magnetic particles were fluidized along with the non-magnetic particles.
Further, from studies at New Jersey Institute of Technology (NJIT), it has been shown that a magnetically assisted impaction coating (MAIC) process may be an effective method for providing the extra force needed to break up the dense phase or layer of particles. The MAIC process has been successfully used as a dry coating method. The MAIC process utilizes an oscillating magnetic field to accelerate magnetic particles thereby providing collisions between particles and the walls of the apparatus. Each of the foregoing techniques are directed to the use of a magnetic field with magnets for accomplishing certain processes, but none of the techniques are directed to fluidization of extreme Geldart C particles, in particular, nano-powders.
At a low sound frequency, typically from 50 to 400 Hz, and a high sound pressure level, typically above 110 dB, sound waves have been shown to improve the fluidization of fine particles, which otherwise showed intense channeling or slugging rather than fluidization [Morse, Sonic energy in granular solid fluidization, Ind. Eng. Chem., 47 (6) (1955) 1170-1175]. Standing waves are generated in the experimental column and at a fixed sound pressure level, sound assisted fluidization can only occur within a certain range of low sound frequencies. Channeling has been found above and below this frequency range [Russo et al., The influence of the frequency of acoustic waves on sound-assisted fluidization of beds offine particles, Powder Technology, 82 (1995) 219-230]. At the natural frequency of the bed of micron sized particles, high intensity sound waves have been found to lead to reductions in both the minimum bubbling velocity and the minimum fluidization velocities [Levy et al., Effect of an acoustic field on bubbling in a gas fluidized bed, Powder Technology, 90 (1997) 53-57]. The literature also shows that an increase in sound pressure level may also yield a decrease in bed expansion, an increase in bubble frequency and an increase in bubble size, and that high intensity sound can also effectively reduce the elutriation of fine particles [Chirone et al., Bubbling fluidization of a cohesive powder in an acoustic field, Fluidization VII, 1992, 545-553]. To date, the reported research has been directed to sound-assisted fluidization of micron or sub-micron sized particles. No results have been reported on the effects of sound on the fluidization of nanoparticle agglomerates.
Thus, despite efforts to date, a need remains for systems and methods/processes that provide for effective fluidization of nanoparticles. A further need remains for systems and processes that uniformly fluidize a bed of nanoparticles. Also needed are systems and processes for nanoparticle fluidization that function without forming a dense layer of agglomerates. Additionally, fluidization systems and processes that minimize powder loss while fluidizing nanoparticles are needed. It is a further need to determine characteristics of nanoparticle agglomerates and to use such characteristics in enhancing fluidization effectiveness.